Understanding the $lookup Operator

• The $lookup operator in MongoDB allows us to perform a left outer join between documents from two collections.
• This means we can perform the query of documents from one collection with data from another collection based on a specified matching condition.
• It’s useful for combining data from different collections for analysis or reporting purposes.
• The operator can handle cases where there are no matching documents in the foreign collection, providing flexibility in data retrieval.

Syntax:

The basic syntax of the $lookup operator is as follows:

{
  $lookup: {
    from: <foreignCollection>,
    localField: <fieldInInputDocument>,
    foreignField: <fieldInForeignDocument>,
    as: <outputArrayField>
  }
}

Explanation:

• from: The name of the foreign collection to join with.
• localField: The field from the input documents that will be used for matching.
• foreignField: The field from the foreign collection that will be used for matching.
• as: The name of the output array field that will contain the joined documents.

Let’s set up an Environment:

To understand MongoDB Aggregation $lookup we need a collection and some documents on which we will perform various operations and queries. Here we will consider a collection called orders and customers which contains various information shown as below:

Collection: orders

[
  {
    "_id": ObjectId("60f9d7ac345b7c9df348a86e"),
    "order_number": "ORD001",
    "customer_id": ObjectId("60f9d7ac345b7c9df348a86d"),
    "customer_details": [
      {
        "_id": ObjectId("60f9d7ac345b7c9df348a86d"),
        "name": "John Doe",
        "email": "john@example.com"
      }
    ]
  },
  {
    "_id": ObjectId("60f9d7ac345b7c9df348a86f"),
    "order_number": "ORD002",
    "customer_id": ObjectId("60f9d7ac345b7c9df348a86d"),
    "customer_details": [
      {
        "_id": ObjectId("60f9d7ac345b7c9df348a86d"),
        "name": "John Doe",
        "email": "john@example.com"
      }
    ]
  },
  {
    "_id": ObjectId("60f9d7ac345b7c9df348a870"),
    "order_number": "ORD003",
    "customer_id": ObjectId("60f9d7ac345b7c9df348a86e"),
    "customer_details": [
      {
        "_id": ObjectId("60f9d7ac345b7c9df348a86e"),
        "name": "Alice Smith",
        "email": "alice@example.com"
      }
    ]
  },
  {
    "_id": ObjectId("60f9d7ac345b7c9df348a871"),
    "order_number": "ORD004",
    "customer_id": ObjectId("60f9d7ac345b7c9df348a86f"),
    "customer_details": [
      {
        "_id": ObjectId("60f9d7ac345b7c9df348a86f"),
        "name": "Bob Johnson",
        "email": "bob@example.com"
      }
    ]
  },
  {
    "_id": ObjectId("60f9d7ac345b7c9df348a872"),
    "order_number": "ORD005",
    "customer_id": ObjectId("60f9d7ac345b7c9df348a86f"),
    "customer_details": [
      {
        "_id": ObjectId("60f9d7ac345b7c9df348a86f"),
        "name": "Bob Johnson",
        "email": "bob@example.com"
      }
    ]
  }
]

Collection: customers

[
  {
    "_id": ObjectId("60f9d7ac345b7c9df348a86d"),
    "name": "John Doe",
    "email": "john@example.com"
  },
  {
    "_id": ObjectId("60f9d7ac345b7c9df348a86e"),
    "name": "Alice Smith",
    "email": "alice@example.com"
  },
  {
    "_id": ObjectId("60f9d7ac345b7c9df348a86f"),
    "name": "Bob Johnson",
    "email": "bob@example.com"
  }
]

The $lookup operator in MongoDB is a powerful tool for performing join-like operations between documents from two collections. It allows us to perform join operations which are defined by documents from one collection with data from another collection based on a specified matching condition.

In this article, We will learn about MongoDB Aggregation $lookup along with multiple examples in detail.